According to the daily stage-discharge data during 1980-2015 collected from Taojiang and Yiyang hydrological stations, the trend and abrupt change of water level and flow discharge in the tail of Zishui River are analyzed. On this premise, the causes and influences of aforementioned changes are explored. Results reveal that runoff in the tail of Zishui River fluctuated periodically during the past 36 years with alternating flood and dry periods; ever since the dry period from 2000, runoff has been decreasing overall with an insignificant trend. In the meantime, affected by riverbed downcutting and water level drawdown in Xiangshui flood channel induced by disorderly dredging, water level in the tail of Zishui river declined sharply, and the mean value changed abruptly by 1.72 m at Taojiang station and 0.80 m at Yiyang station. In addition, the abrupt change of water level has brought about evident changes in stage-discharge relation together with the reciprocal relation between river and lake. By replenishing into river unidirectionally, lake water has alleviated the further drawdown of water level in the Zishui river.